The mainstream of Christianity has always held the cross of Christ to be central. David Gibson tells in his Assumed Evangelicalism:
In 1919, Trinity Great Court in Cambridge saw a meeting between Rollo Pelly, the Secretary of the liberal Student Christian Movement, and Daniel Dick and Norman Grubb (President and Secretary of the evangelical Cambridge Inter-Collegiate Christian Union). The meeting was to discuss the re-unification of the two movements that had split in 1910. Norman Grubb's account of the meeting is infamous:
After an hour's talk, I asked Rollo point blank, 'Does the SCM put the atoning blood of Jesus Christ central?' He hesitated, and then said, 'Well, we acknowledge it, but not necessarily central.' Dan Dick and I then said that this settled the matter for us in the CICCU. We could never join something that did not maintain the atoning blood of Jesus Christ at its centre; and we parted company.'
The cross is central because without it there is nothing to Christianity. And it's no surprise that Charles Simeon famously said
that a nominal Christian is happy to prove the importance of the crucified redeemer.... but the true Christian delights in the cross, rejoices in it, glories in it and shudders at the thought of glorying in anything else. (cited from John Piper's biography of Simeon)
The cross is so important because of it's effect. The cross is so important because of it's purpose.
1. For our sins.Jesus stepped into this world to die because of our sins. His death had a specific purpose. Not merely symbolic but effective. Effective bear the curse of law and sin. More on that in Galatians 3.
2. To deliver us.
The cross is about freedom. It's about rescue. It's about liberation. From what? From "this present evil age". You might read that and think Jesus died to liberate Christians from materialism and pornography and rolling-24-hour news and a million other present concerns. But, looking at the letter itself we get a clearer picture.
Think of Paul. Paul used to be a slave, 1v13. He tells us what he used to be before he was a Christian, now he has been delivered from that - hence his old way of life was slavery. Moreover we know it because he says that in his old way of life he persecuted the church. We know from 4v29-30 that the people who persecute the church were slaves. What was Paul's slavery? Zeal for the Jewish law. Not the classic "present evil age" we might imagine!
Think of the Galatians. 4v8-11 speaks of their prior worship of "not gods". Presumably they were classic pagan idolators, citizens of the Roman empire who worshipped all kinds of idols. That fits more with a classic Christian definition of the present evil age. But Galatians requires both - slavery to sinfully-indulgent idols and slavery to self-righteous idols. The death of Jesus secures deliverance from both of these.
How? By being curse bearing and (as we'll see at the end of Galatians 2 and 6) by killing us with Jesus, so that we can rise to new life that is lived in union with Jesus, that is in the risen Jesus. Calvin said
"we must understand that as long as Christ remains outside of us, and we are separated from him, all that he has suffered and done for the salvation of the human race remains useless and of no value to us... [so let us] climb higher and examine the secret energy of the Spirit, by which we come to enjoy Christ and all his benefits”. But, we're getting ahead of ourselves.
The cross of Christ is about dealing with sin and setting us free. Push that off centrestage and you've got something other than Christianity.